Conductive field devices for measuring limit level in liquids have been known for many years. In these field devices, the ohmic resistance, or conductance, between a conductive probe projecting into the container and the container wall, or between two electrodes of a probe, is measured. If a conductive medium forms an electrical connection between the conductive probe and the container wall, or between both electrodes of a probe, as the case may be, then the measured ohmic resistance is noticeably lowered.
The simplest possibility for measuring this ohmic resistance is to place a DC voltage on the probe circuit, and then measure the current flowing through the probe. This method, however, has two disadvantages: one is that the constantly flowing DC current can lead to corrosion of the conductive probe rods or the container wall, and the other is that the conductive probe rods and the container wall can form a galvanic element, and thus produce a current flow which is superimposed on the measuring current and, consequently, corrupts the measurement results. For these reasons, it is more sensible to measure with an alternating voltage, whereby a complete DC voltage separation of the measuring circuit from the probe circuit, and thus from the process, becomes possible by means of capacitors. Until now, circuits of this kind of have been implemented using comparatively complex analog technology.